Process for producing wiring circuit board

ABSTRACT

The invention provides a process for producing a wiring circuit board, which comprises the steps of: (A) forming a conductor layer of a predetermined pattern on an insulating layer; (B) forming a photosensitive solder resist layer on the insulating layer and the patterned conductor layer formed on the insulating layer; (C) disposing a transparent protective film on the photosensitive solder resist layer; and (D) exposing the photosensitive solder resist layer to a light through the transparent protective film.

FIELD OF THE INVENTION

The present invention relates to a process for producing a wiringcircuit board having a solder resist.

BACKGROUND OF THE INVENTION

In steps for producing wiring circuit boards such as, e.g., flexiblewiring circuit boards, a solder resist layer (covering insulating layer)is formed in order to protect the wiring pattern (conductor layer) andprevent solder bridging between connection terminals.

Screen printing has hitherto been used for forming solder resist layers.However, with the trend toward higher wiring pattern densities,photographic techniques capable of coping with finer wiring pitches(smaller spaces between wiring patterns) have recently come to begenerally used in place of screen printing.

FIGS. 4 are diagrammatic sectional views illustrating an example ofprocedures of a conventional process for producing a wiring circuitboard.

First, a conductor layer 90 b having a predetermined pattern is formedon an insulating layer 90 a as shown in FIG. 4(a). Thereafter, asolution of a photosensitive solder resist is applied on the surface ofthe insulating layer 90 a including the conductor layer 90 b. Thesolution of a photosensitive solder resist on the insulating layer 90 ais then dried in a circulating hot-air oven to form a photosensitivesolder resist layer SR as shown in FIG. 4(b).

Subsequently, as shown in FIG. 4(c), the photosensitive solder resistlayer SR is irradiated with ultraviolet ray (UV) through a mask MKhaving a predetermined pattern. Thus, the photosensitive solder resistlayer SR is selectively exposed to the light.

The exposed solder resist layer SR is then developed as shown in FIG.4(d) to form a photosensitive solder resist layer SR having apredetermined pattern. The photosensitive solder resist layer SR is thensubjected to a curing treatment to thereby give a wiring circuit board900.

In FIG. 4(d), the central conductor layer 90 b corresponds to, forexample, a wiring part and the other two conductor layers 90 bcorresponds to, for example, connection terminals.

The photosensitive solder resist generally contains low-molecularingredients such as, e.g., a polymerization initiator and a sensitizerso as to impart photosensitivity. However, due to the presence of thelow-molecular ingredients such as the polymerization initiator and thesensitizer, the unexposed surface of the photosensitive solder resistlayer SR has tackiness at room temperature even after drying in acirculating hot-air oven.

Consequently, in the conventional process for producing a wiring circuitboard 900, it is impossible, after the drying of the photosensitivesolder resist layer SR in a circulating hot-air oven, to stack up cutsheets of the resultant intermediate product or to wind the intermediateproduct into a roll. This is because if the intermediate product issuperposed one on another or wound into a roll, the photosensitivesolder resist layer SR, which has tackiness, undesirably adheres to theoverlying insulating layer 90 a.

On the other hand, in processes for producing a flexible wiring circuitboard having flexibility, a roll-to-roll operation is generallyconducted frequently.

In the roll-to-roll operation, a long sheet (long sheet substrate) whichhas been wound around a roller is drawn out from the roller, subjectedto predetermined treatments or processings, and then wound around aroller. This operation improves the efficiency of production of flexiblewiring circuit boards and can realize a reduction in production cost.

However, if the above-mentioned intermediate product is wound into aroll, the photosensitive solder resist layer SR comes into contact withthe insulating layer 90 a and hence adheres thereto because the surfaceof the photosensitive solder resist layer SR has tackiness at ordinarytemperature. Therefore, in the case where the above-mentionedphotosensitive solder resist layer SR is used, the roll-to-rolloperation cannot be conducted.

A technique for overcoming the problem described above has been proposedwhich comprises applying a release agent, e.g., a silicone resin, on thephotosensitive solder resist layer SR to form a coating in order toavoid the tackiness of the photosensitive solder resist layer SR (see,for example, patent document 1 specified below).

In this case, even when the photosensitive solder resist layer SR comesinto contact with the insulating layer 10 a, the two layers are easilyseparated from each other by virtue of the release agent.

Patent Document 1: JP 05-243715A

However, the technique in which a coating film of a release agent, e.g.,a silicone resin, is formed on the photosensitive solder resist layer SRhas a drawback that there are cases where the application of the releaseagent on the photosensitive solder resist layer SR gives coatingunevenness. Since the photosensitive solder resist layer SR is exposedto light through the release agent, the coating unevenness causes, inthe exposure shown in FIG. 4(c), the photosensitive solder resist layerSR to suffer exposure unevenness. As a result, the exposed solder resistlayer SR comes to have, for example, dissolution rate unevenness withrespect to the developing solution in the development step thereof.There are hence cases where the resultant solder resist layer SR hasthickness unevenness.

Furthermore, there are cases where uneven application of the releaseagent results in insufficient drying of the release agent itself andthis results in sticking in intermediate product superposition.

In addition, since a coating operation and a drying operation eachshould be conducted twice, a large and expensive apparatus is necessaryin the case where a continuous processing operation is to be conductedwith a coating machine comprising the corresponding film coaters anddryers. Use of such an apparatus undesirably results in increasedproduction cost.

SUMMARY OF THE INVENTION

An object of the invention is to provide a process for wiring circuitboard production by which a wiring circuit board having high reliabilitycan be produced at low cost.

Other objects and effects of the invention will become apparent from thefollowing description.

The invention provides a process for producing a wiring circuit boardwhich comprises the steps of:

-   -   (A) forming a conductor layer of a predetermined pattern on an        insulating layer;    -   (B) forming a photosensitive solder resist layer on the        insulating layer and the patterned conductor layer formed on the        insulating layer;    -   (C) disposing a transparent protective film on the        photosensitive solder resist layer; and    -   (D) exposing the photosensitive solder resist layer to a light        through the transparent protective film.

In the process of the invention for producing a wiring circuit board, atransparent protective film is disposed on a photosensitive solderresist layer. Namely, the photosensitive solder resist layer is coveredwith the transparent protective film. Because of this, even whenintermediate products in which the photosensitive solder resist layerhas not undergone a curing treatment are stacked up, the photosensitivesolder resist layer of the underlying intermediate product is preventedfrom sticking to the insulating layer of the overlying intermediateproduct. In this case, the term “intermediate product” means amultilayered structure comprising an insulating layer, a conductorlayer, a photosensitive (or exposed) solder resist layer, and atransparent protective film.

Furthermore, since the transparent protective film is evenly disposed onthe photosensitive solder resist layer, the transparent protective filmdoes not cause exposure unevenness when the photosensitive solder resistlayer is exposed to light. Consequently, even when the conductor layerhas a finer pattern, the exposure of the photosensitive solder resistlayer can be conducted with satisfactory precision. As a result, awiring circuit board having high reliability can be produced.

The process may further comprises:

-   -   (E) peeling the transparent protective film from the exposed        solder resist layer; and    -   (F) developing the exposed solder resist layer.

In this case, the transparent protective film is peeled from the exposedsolder resist layer and the exposed solder resist layer is thendeveloped. As a result, a solder resist layer having a predeterminedpattern can be formed.

The insulating layer and the transparent protective film each may be along sheet and the process may further comprise, after the above step(C) or (D), winding a long sheet-shaped multilayered structurecomprising the insulating layer, the conductor layer, the photosensitiveor exposed solder resist layer and the transparent protective film intoa roll.

In this case, the long sheet-shaped multilayered structure comprisingthe insulating layer in a long sheet form, conductor layer,photosensitive solder resist layer and transparent protective film iswound into a roll after the transparent protective film-disposing step(C) or after the photosensitive solder resist layer-exposing step (D)

This embodiment improves the efficiency of wiring circuit boardproduction and attains a reduction in the cost of wiring circuit boards.

The process may further comprise preparing a roll of a long sheet-shapedsubstrate comprising an insulating layer, optionally having thereon aconductor layer, so that the steps (A) to (C) are successively carriedout with respect to the long sheet-shaped substrate drawn out from theroll thereof.

In this case, a roll of a long sheet-shaped substrate comprising aninsulating layer, optionally having thereon a conductor layer isprepared first. The long sheet-shaped substrate is drawn out from theroll and subjected successively to the patterned conductor layer-formingstep (A), the photosensitive solder resist layer-forming step (B), andthe transparent protective film-disposing step (C). As a result, theefficiency of wiring circuit board production is improved and areduction in the cost of wiring circuit boards is attained.

In the process of the invention for producing a wiring circuit board, atransparent protective film is disposed on a photosensitive solderresist layer. Namely, the photosensitive solder resist layer is coveredwith a transparent protective film. Therefore, even when intermediateproducts in which the photosensitive solder resist layer has notundergone a curing treatment are stacked up, the photosensitive solderresist layer of the underlying intermediate product is prevented fromsticking to the insulating layer of the overlying intermediate product.In this case, the term “intermediate product” means a multilayeredstructure comprising an insulating layer, a conductor layer, aphotosensitive (or exposed) solder resist layer, and a transparentprotective film.

Furthermore, since the transparent protective film is evenly disposed onthe photosensitive solder resist layer, the transparent protective filmdoes not cause exposure unevenness when the photosensitive solder resistlayer is exposed to light. Consequently, even when the conductor layerhas a finer pattern, the exposure of the photosensitive solder resistlayer can be conducted with satisfactory precision. As a result, awiring circuit board having high reliability can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) to 1(c) are diagrammatic sectional views illustrating stepsof one embodiment of the process for producing a flexible wiring circuitboard of the invention.

FIGS. 2(d) to 2(e) are diagrammatic sectional views illustrating stepsof the embodiment of the process for producing a flexible wiring circuitboard of the invention.

FIG. 3 is a diagrammatic view illustrating an example of the apparatusfor producing a flexible wiring circuit board by a roll-to-rolloperation according to one embodiment of the invention.

FIGS. 4(a) to 4(d) are diagrammatic sectional views illustrating anexample of procedures of the conventional process for producing a wiringcircuit board.

The reference numerals and signs used in the drawings denote thefollowings, respectively.

10 a: Insulating layer

10 b: Conductor layer

300: Roll-to-roll apparatus

310: Unwinding roller

321: Conductor layer-forming device

322: Solder resist-coating device

323: Exposing device

330: Film-feeding roller

331: Laminating roller

340: Wind-up roller

SR: Photosensitive solder resist layer

Fi: Transparent protective film

TU: Intermediate sheet

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the process for producing a flexible wiring circuitboard according to the invention will be explained below by reference toFIGS. 1 to 3.

FIGS. 1 and 2 are diagrammatic sectional views illustrating steps of oneembodiment of the process for producing a flexible wiring circuit boardof the invention.

First, a conductor layer 10 b having a predetermined pattern is formedon an insulating layer 10 a as shown in FIG. 1(a) (conductor layerformation). Thus, a wiring pattern of the conductor layer 10 b isformed. For forming the patterned conductor layer 10 b, any of thesubtractive method, semi-additive method, and additive method may beused.

The subtractive method is a technique in which a conductor layer isformed beforehand and unnecessary parts of the conductor layer areremoved by etching to thereby form a conductor layer 10 b having apredetermined pattern. The additive method is a technique in which aconductor layer 10 b having a predetermined pattern is formed throughdeposition by, e.g., plating. The semi-additive method is a techniquecomprising a combination of the subtractive method and the additivemethod.

The insulating layer 10 a is not particularly limited as long as it hasflexibility and electrical insulating properties. However, theinsulating layer 10 a preferably is made of, e.g., a polyimide, whichhas chemical resistance and heat resistance. The thickness of theinsulating layer 10 a is preferably in the range of 5-100 μm, morepreferably 10-30 μm.

The insulating layer 10 a is preferably a long sheet (long sheet-shapedsubstrate). In the case where the insulating layer 10 a is a long sheet,it is preferred to conduct a roll-to-roll operation. In the roll-to-rolloperation, the long sheet which has been wound around a roller is drawnout from the roller, subjected to predetermined treatments orprocessings, and then wound around a roller. This roll-to-roll operationwill be described later in detail.

The conductor layer 10 b is not particularly limited as long as it haselectrical conductivity. However, the conductor layer 10 b is preferablymade of copper. The thickness of the conductor layer 10 b is preferablyin the range of 3-50 μm, more preferably 5-20 μm.

Subsequently, a solution of a photosensitive solder resist is applied onthe surface of the insulating layer 10 a including the conductor layer10 b (solder resist application). The applied photosensitive solderresist solution is then dried in, e.g., a circulating hot-air oven.Thus, a photosensitive solder resist layer SR is formed over theinsulating layer 10 a as shown in FIG. 1(b). The surface of thisphotosensitive solder resist layer SR has tackiness even after thedrying.

The material of the photosensitive solder resist is not particularlylimited. Typical examples thereof include materials such as acrylic,urethane, epoxy, and polyimide materials. Any of these may be used. Sucha photosensitive solder resist is dissolved in a solvent to prepare asolution.

The drying of the photosensitive solder resist solution applied on thesurface of the insulating layer 10 a is preferably conducted at atemperature of 60-100° C.

The thickness of the photosensitive solder resist layer SR is preferablyin the range of 3-50 μm, more preferably 5-30 μm, most preferably 8-20μm.

Thereafter, as shown in FIG. 1(c), a transparent protective film Fi in asheet form is superposed on the photosensitive solder resist layer SR(superposition of transparent protective film). Thus, an intermediatesheet TU which comprises the insulating layer 10 a, conductor layer 10b, photosensitive solder resist layer SR and transparent protective filmFi is produced. In this embodiment, the superposition of the transparentprotective film Fi on the photosensitive solder resist layer SR isconducted, for example, by the roll-to-roll operation which will bedescribed later. However, methods for the superposition are not limitedthereto.

The transparent protective film Fi is not particularly limited as longas it is capable of transmitting a light having a wavelength to whichthe photosensitive solder resist layer SR has a sensitivity. Withrespect to the light transmittance, the transparent protective film Fipreferably has such a light transmittance that it transmits at least 80%of a light having a wavelength in the range of 100-800 nm. Morepreferably, the transparent protective film Fi has such a lighttransmittance that it transmits at least 80% of a light having awavelength in the range of 350-450 nm.

Furthermore, the transparent protective film Fi preferably has highlight transmittance especially to lights having wavelengths of i-line(365 nm), h-line (405 nm), and g-line (436 nm).

When the transparent protective film Fi has such transmissioncharacteristics, a light necessary for allowing the photosensitivesolder resist layer SR to photoreact, e.g., ultraviolet ray (wavelength,about 1-400 nm), passes through the transparent protective layer. Thephotosensitive solder resist layer SR can hence be easily allowed tophotoreact, and exposure can be satisfactorily conducted.

Examples of the material of the transparent protective film Fi includepolyethylene and polyesters. In the case where a polyester is used asthe material of the transparent protective film Fi, a cost reduction isrealized because polyesters are inexpensive.

In the case where polyethylene is used as the material of thetransparent protective film Fi, air bubble inclusion is less apt tooccur in superposing the protective film Fi on the photosensitive solderresist layer SR because polyethylene is flexible and well conforms tothe surface irregularities due to the conductor layer 10 b of apredetermined pattern. Use of polyethylene hence enables exposure to besatisfactorily conducted.

The thickness of the transparent protective film Fi is preferably in therange of 5-50 μm, more preferably 10-30 μm.

In this embodiment, when intermediate sheets TU in which thephotosensitive solder resist layer SR has not undergone the curingtreatment described later are stacked up, then the photosensitive solderresist layer SR, which has tackiness, is prevented from sticking to theinsulating layer 10 a because the transparent protective film Fi issuperposed on the photosensitive solder resist layer SR.

Subsequently, as shown in FIG. 2(d), the photosensitive solder resistlayer SR is irradiated with ultraviolet ray UV through a mask MK havinga predetermined pattern (solder resist exposure), thereby beingselectively exposed to the ultraviolet ray UV through the transparentprotective film Fi.

Referring to conditions for this exposure, the exposure amount(irradiation energy per unit area) is preferably 100-800 mJ/cm². Theexposure period is appropriately controlled according to thetemperature, properties of the photosensitive solder resist layer SR,the wavelength of the ultraviolet ray UV to be used for irradiation,etc. Thus, the photosensitive solder resist layer SR is allowed tophotoreact in accordance with the predetermined pattern of the mask MK.

Thereafter, the transparent protective film Fi on the exposed solderresist layer SR is peeled off (peeling of transparent protective film).

The exposed solder resist layer SR is immersed in a developing solution,whereby the exposed solder resist layer SR is developed to form apredetermined pattern corresponding to the mask MK (solder resistdevelopment).

As the developing solution, there can be used an aqueous alkali solutionin general use, such as, e.g., an aqueous solution of sodium carbonate,sodium hydroxide, or potassium hydroxide. The concentration of thedeveloping solution is preferably in the range of, e.g., 0.3-5%. Thetemperature for the development is preferably in the range of 20-60° C.

Finally, the developed solder resist layer SR is heated (solder resistcuring). By heating, the exposed solder resist layer SR undergoes acuring reaction and its tackiness disappears. Thus, a flexible wiringcircuit board 100 according to this embodiment is completed as shown inFIG. 2(e). The temperature for heating the developed solder resist layerSR is preferably in the range of 120-200° C.

In FIG. 2(e), the central conductor layer 10 b corresponds, for example,to a wiring part and the other two conductor layers 10 b corresponds,for example, to connection terminals.

An explanation is then given on the roll-to-roll operation. FIG. 3 is adiagrammatic view illustrating an example of the apparatus usable forproducing a flexible wiring circuit board 100 by a roll-to-rolloperation according to this embodiment. Hereinafter, the apparatus shownin FIG. 3 is referred to as a roll-to-roll apparatus.

With the roll-to-roll apparatus 300 shown in FIG. 3, the conductor layerformation, solder resist application, and superposition of transparentprotective film described above are conducted. The solder resistexposure may be further conducted as needed.

The roll-to-roll apparatus 300 comprises a conductor layer-formingdevice 321, a solder resist-coating device 322, an unwinding roller 310,a film-feeding roller 330, a pair of laminating rollers 331, and awind-up roller 340. In the case where the solder resist exposure isconducted in the roll-to-roll apparatus 300, an exposing device 323(indicated by the broken lines in FIG. 3) is also included in theconstitution of the apparatus.

Around the unwinding roller 310 in FIG. 3, there has been wound a longsheet-shaped insulating layer 10 a The unwinding roller 310 rotates inthe direction indicated by the arrow R1 (the direction opposite to thedirection of winding of the insulating layer 10 a). The insulating layer10 a is drawn out as a result of the rotation, passes through theconductor layer-forming device 321, solder resist-coating device 322,and pair of laminating rollers 331, and is then sent to the wind-uproller 340. This wind-up roller 340 rotates in the direction indicatedby the arrow R2 (the same direction as that indicated by R1). As aresult, the insulating layer 10 a on which various layers have beenformed is wound around the wind-up roller 340.

In the conductor layer-forming device 321, the conductor layer formationis conducted. As a result, a conductor layer 10 b of a predeterminedpattern is formed on the insulating layer 10 a. The insulating layer 10a on which the patterned conductor layer 10 b has been formed is thensent to the solder resist-coating device 322.

In the solder resist-coating device 322, the solder resist applicationis conducted. Here, a solution of a photosensitive solder resist isapplied on the surface of the insulating layer 10 a including theconductor layer 10 b, and the photosensitive solder resist applied isdried. As a result, a photosensitive solder resist layer SR is formed.The insulating layer 10 a over which the photosensitive solder resistlayer SR has been formed is then sent to the pair of laminating rollers331.

Around the film-feeding roller 330, there has been wound a transparentprotective film Fi. The film-feeding roller 330 rotates in the directionindicated by the arrow R3 to thereby feed the transparent protectivefilm Fi to one of the pair of laminating rollers 331.

At the pair of laminating rollers 331, the superposition of thetransparent protective film is conducted. In this processing, thetransparent protective film Fi is laminated to the photosensitive solderresist layer SR formed by the solder resist-coating device 322. As aresult, an intermediate sheet TU comprising the insulating layer 10 a,conductor layer 10 b, photosensitive solder resist layer SR, andtransparent protective film Fi as shown in FIG. 1(c) is formed. Thisintermediate sheet TU is wound around the wind-up roller 340.

In the case where the solder resist exposure is conducted in theroll-to-roll apparatus 300, the intermediate sheet TU is sent from thelaminating rollers 331 to the exposing device 323. The intermediatesheet TU sent to the exposing device 323 is subjected to the solderresist exposure. The intermediate sheet TU which has undergone thesolder resist exposure is sent to the wind-up roller 340 and woundaround the wind-up roller 340.

As described above, in the roll-to-roll apparatus 300, varioustreatments or processings are successively conducted during when thelong sheet-shaped insulating layer 10 a is sent from the unwindingroller 310 to the wind-up roller 340. Consequently, the roll-to-rolloperation not only improves the efficiency of wiring circuit boardproduction but also attains a reduction in the cost of wiring circuitboards.

In the foregoing embodiment, the formation of conductor layer, solderresist application, and superposition of a transparent protective filmare conducted through a roll-to-roll operation using the roll-to-rollapparatus 300 shown in FIG. 3. However, it is also possible to useanother roll-to-roll apparatus so as to subject the intermediate sheetTU wound around the wind-up roller 340 to solder resist exposure,peeling of the transparent protective film, solder resist development,and solder resist curing through a roll-to-roll operation. This processfurther improves the efficiency of wiring circuit board production andattains a further reduction in the cost of wiring circuit boards.

In the foregoing embodiment, a roll-to-roll operation is made possibleby superposition of the transparent protective film Fi on thephotosensitive (or exposed) solder resist layer SRhaving tackiness.Consequently, according to this embodiment of the process for producinga flexible wiring circuit board 100, a high production efficiency and acost reduction can be attained.

Furthermore, because a transparent protective film Fi in a sheet form isused, the transparent protective film Fi can be evenly superposed on thephotosensitive solder resist layer SR. As a result, the transparentprotective film Fi does not cause exposure unevenness when the solderresist layer is exposed to light and, hence, development unevenness doesnot occur during the solder resist development. Consequently, even whenthe conductor layer has a finer wiring pattern, a solder resist layer SRcan be formed with satisfactory precision.

In the foregoing embodiment, the insulating layer 10 a, conductor layer10 b, and photosensitive solder resist layer SR correspond to theinsulating layer, conductor layer, and photosensitive solder resistlayer, respectively.

Further, the transparent protective film Fi, intermediate sheet TU,insulating layer 10 a, and unwinding roller 310 correspond to thetransparent protective film, long sheet-shaped multilayered structure,long sheet-shaped insulating layer substrate, and roller, respectively.

EXAMPLES

The present invention will be illustrated in greater detail withreference to the following Examples, but the invention should not beconstrued as being limited thereto.

A flexible wiring circuit board 100 was produced in accordance with theforegoing embodiment of the process for producing a flexible wiringcircuit board 100. The procedure for production is as follows.

First, a long sheet-shaped two-layer substrate composed of a 25 μm-thickpolyimide and a 12 μm-thick copper foil directly formed thereon(Espanex, manufactured by Nippon Steel Chemical Co., Ltd.) was prepared.This two-layer sheet substrate was wound into a roll.

In this Example, the polyimide corresponds to the insulating layer 10 ain FIGS. 1 and the copper foil (after being patterned) corresponds tothe conductor layer 10 b in FIGS. 1.

The following procedure comprises successive processings conducted by acontinuous roll-to-roll operation.

The copper foil of the two-layer sheet substrate was etched so as toresult in a predetermined pattern in order to obtain a conductor layerlob of the predetermined pattern (subtractive method). Thus, a conductorlayer 10 b of the predetermined pattern was formed.

Subsequently, a photosensitive solder resist solution (NPR-80,manufactured by Nippon Polytech Corp.) was applied on the surface of thepolyimide (insulating layer 10 a) including the patterned copper foil(conductor layer 10 b) in such an amount as to give a coating thicknessof 20 μm. This application of the solution was conducted with a coatingdevice 322. Thereafter, the coating was continuously dried in a dryingoven. The drying was conducted in an 80° C. atmosphere. Thus, aphotosensitive solder resist layer SR having tackiness was formed. Thedrying oven was located downstream from the coating device 322 in FIG.3.

A polyester film having a thickness of 16 μm (HTF01, manufactured byTeijin Ltd.; thickness, 16 μm) was laminated onto the photosensitivesolder resist layer SR with laminating rollers 331. This polyester filmcorresponds to the transparent protective film Fi in FIG. 1(c). Thepolyester film had a light transmittance of about 87% as measured at awavelength of 408 nm.

The resultant intermediate sheet UV composed of the two-layer substrate,photosensitive solder resist layer SR, and polyester film was wound by awind-up roller 340.

Subsequently, the polyester film and photosensitive solder resist layerSR of the wound intermediate sheet TU were irradiated with a lighthaving a wavelength of 365 nm through a mask having a predeterminedpattern. Thus, a series of successive processings was completed.

Thereafter, the polyester film was peeled from the wound intermediatesheet TU, and the exposed solder resist layer SR was developed with adeveloping solution. This development was conducted with a 0.7% aqueoussodium carbonate solution having a temperature of 25° C.

Finally, the developed solder resist layer SR was subjected to a curingtreatment. In this curing treatment, the solder resist layer SR washeated in a 150° C. atmosphere for 60 minutes. Thus, a flexible wiringcircuit board 100 according to the Example was obtained.

In this Example described above, a polyester film (transparentprotective film Fi) was laminated to the photosensitive solder resistlayer SR, which had tackiness. Because of this, when the intermediatesheet TU was wound into a roll, the photosensitive solder resist layerSR which had not undergone a curing treatment did not adhere to thepolyimide (insulating layer 10 a). As a result, after the formation ofthe photosensitive solder resist layer SR, the exposure, development,and curing treatment of the solder resist could be conducted smoothly.

The invention is useful in producing a wiring circuit board having asolder resist.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

This application is based on Japanese patent application No. 2003-315634filed Sep. 8, 2003, the contents thereof being herein incorporated byreference.

1. A process for producing a wiring circuit board, which comprises thesteps of: (A) forming a conductor layer of a predetermined pattern on aninsulating layer; (B) forming a photosensitive solder resist layer onthe insulating layer and the patterned conductor layer formed on theinsulating layer; (C) disposing a transparent protective film on thephotosensitive solder resist layer; and (D) exposing the photosensitivesolder resist layer to a light through the transparent protective film.2. The process of claim 1, which further comprises: (E) peeling thetransparent protective film from the exposed solder resist layer; and(F) developing the exposed solder resist layer.
 3. The process of claim1, wherein the insulating layer and the transparent protective film eachis a long sheet, and wherein the process further comprises, after thestep (C) or (D), winding a long sheet-shaped multilayered structurecomprising the insulating layer, the conductor layer, the photosensitiveor exposed solder resist layer and the transparent protective film, intoa roll.
 4. The process of claim 1, wherein the process further comprisespreparing a roll of a long sheet-shaped substrate comprising aninsulating layer, optionally having thereon a conductor layer, andwherein at least the steps (A) to (C) are successively carried out withrespect to the long sheet-shaped substrate drawn out from the rollthereof.
 5. The process of claim 4, wherein the steps (A) to (D) aresuccessively carried out with respect to the long sheet-shaped substratedrawn out from the roll thereof.
 6. The process of claim 1, wherein thetransparent protective film has a thickness of 5 to 50 μm.
 7. Theprocess of claim 1, wherein the transparent protective film has a lighttransmittance of at least 80% with respect to a light having awavelength in the range of 100-800 nm.
 8. The process of claim 1,wherein the transparent protective film comprises at least one ofpolyethylene and polyester.